Sterols are alcoholic substances found in non-saponifiable matter of plants. They are valuable compounds which can be used in the food, in the pharmaceuticals and in the chemical industries.
Sterols can be recovered from tall oil pitch which is a distillation residue obtained from crude tall oil distillation. Crude tall oil (CTO) originates from the black liquor obtained from coniferous trees by extraction in a kraft pulp process. CTO is typically recovered from soap skimming of black liquor and contains high quantities of fatty acid sodium salts, resin acid sodium salts and unsaponifiables and neutral substances which include fatty alcohols, free sterols, steryl esters, and fatty acid esters.
In kraft pulp mills, skimmed soap is collected and acidulated with a mineral acid, such as sulphuric acid, to obtain an oil phase and a water phase. The oil phase, i.e. the crude tall oil, contains free fatty acids, resin acids and unsaponifiables where the amount of unsaponifiables can range from 10 to 35% by weight depending on the species and a quality of coniferous trees used. Crude tall oil is typically dried and distillated at high temperatures under vacuum to yield a light phase, a rosin phase, a fatty acid phase and a pitch phase.
The composition of the tall oil pitch is strongly dependent on the distillation equipment and operating conditions employed in the distillation of tall oil. The residual pitch fraction contains various quantities of fatty and rosin acids and a substantial amount of the original unsaponifiables and neutral substances with concentrated amounts of sterols and steryl esters.
Typically, sterols are obtained from the raw-material by processing methods which comprise a first step of modifying steryl esters and generating free sterols which are then further concentrated into a sterol rich fraction before it is isolated from its impurities to obtain pure sterols.
It is known in the art that sterols can be effectively separated from tall oil pitch by modifying steryl esters in ways which liberate free sterol and fatty acid forms from esterified sterols. Processes are known in the art for saponification of tall oil pitch with an alkali to hydrolyze the fatty acid steryl esters and to release sterols and convert fatty acid to corresponding salts. Sterols are further processed to obtain purified sterols.
Free sterols and fatty acids can be obtained by hydrolyzing reactions. These components can then be separated.
The sterols obtained by the known methods are concentrated by using consecutive evaporations or distillations. The concentration steps commonly comprise evaporation separations carried out in evaporation units, preferably by using technologies in art based on continuous film generating evaporators like Falling Film Evaporator (FFE), Wiped Film Evaporator (WFE), Thin Film Evaporator (TFE), or Short Path Evaporator (SPE).
The sterols can be further purified. Widely used technologies involve the use of solvents: free sterols are isolated and purified using solvent crystallization and filtration processes.
Many of the processing methods discussed above will result in processes which give sterols in high yields. However, at the same time, the processes are relatively complex, and give rise to undesired side streams and other disadvantages which decrease process profitability and sustainability.
Thus, for example, the saponification method is efficient and produces free sterols in high conversion but causes undesirable processing steps afterwards when fatty acid salts/soaps need to be acidified and separated from its water phase before further evaporations. An acidulation step needs to be performed accurately at a certain pH for conditioning pitch soap for phase separation that is normally carried out by decanting. Acidulation generates high quantities of acidic salty waste water and a middle phase sludge which needs to be treated afterwards.
In processes of the above kind, there is still a high tendency for sterols to be re-esterified back to fatty acid esters after the acidulation step. There are also processing difficulties in sterol separation from saponified residuals due to its hardness in hard pitch fraction.
In some methods sterols are fractionated from a reaction medium in which fatty acid esters are first derivatized to the corresponding fatty acid alcohol esters by using various catalytic transesterification methods. Thus, US2013041192 discloses a method for isolating fatty acid alcohol esters for bio fuels. The method has limited applicability for the production free sterols because free sterols are generally obtained at conversion rates of 50 wt-% or less.
All the above features together lead to expensive and difficult processing which is environmentally unsustainable for today's demands.
The literature also describes processing methods for obtaining sterols from other raw material sources. Such sources are side streams from food fats or feed processing i.e. side streams like deodorizer distillate obtained from refining fats and oils. Many of these methods include traditional processing methods, like extractions and distillations, for obtaining sterol rich phases which can be purified with suitable solvents.
There is therefore a need for new methods which are less complex while still reducing the volume of side streams.